Step 1: draft a pattern. This is not hard AT ALL. Simply take 3 sheets of 8 1/2 x 11 inch paper and tape them together on the long edges. Trim 1/2 inch off of one long edge to give you one large 11 x 25 inch pattern piece (that is my green paper). Do this again (yellow paper). After trimming the 1/2 inch off of one edge, fold your pattern in half and design your pocket. The 'wings' on mine are about 4 1/2 inches tall by about 4 1/3 inches long. The top flat bit is about an inch wide. After marking those off, I just connected the dots with a long diagonal line.
Step 3:With right sides together, sew along the top edges of the pocket, leaving the top bit open. Clip the corner where the diagonal meets the straight bit, turn and press. Top stitch over seam. At this point I then top stitched a line right down the middle and down what will become the fold line.
Step 5: Using your top stitching lines as a guide, stitch over them, through all layers. At this point you can add more top stitched lines for pencils and pens if you want to. I added a line approximately one inch over from the fold line.
Step 6: Fold under the seams where you left the opening. Press well. You're going to be top stitching the top and bottom - it's now time to fold the ends (the part with the wings) to the inside. Pin in place, then stitch about 1/4 inch from the top and again at the bottom.